Bifunctional water splitting catalysts and associated methods

ABSTRACT

A catalyst, including a conductive substrate coated with a metal-phosphorus-derived film, where the metal is Manganese, Iron, Cobalt, Nickel, or Copper. In some embodiments, the conductive substrate includes copper, titanium, glassy carbon, fluorine-doped tin oxide, indium-doped tin oxide, tin, nickel, or stainless steel. Methods for producing the catalysts and for hydrogen evolution reactions and oxygen evolution reactions employing the catalysts are also described herein.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/141,081, filed on Mar. 31, 2015, the entirety of which isincorporated by reference.

TECHNICAL FIELD

The present disclosure relates to catalysts and methods for hydrogenand/or oxygen evolution from water. More specifically, it relates tometal-phosphorus-derived film catalysts and methods and applications ofthe same.

BACKGROUND

Electrocatalytic water splitting, which consists of H₂ evolutionreactions (“HER”) and O₂ evolution reactions (“OER”) has attractedincreasing interest in the last few years because of its criticalimportance in the context of renewable energy research. Most efforts inthis field are devoted to developing HER catalysts under strongly acidicconditions for proton-exchange membrane electrolyzers, whereas OERcatalysts operate under strongly basic conditions for alkalineelectrolyzers. Transition-metal chalcogenides, pnictides, carbides,borides, and even metal-free materials have been reported for HERcatalysis in strongly acidic electrolytes. On the other hand, manyinnovative noble-metal-free OER catalysts based on the oxides/hydroxidesof cobalt, nickel, manganese, iron, and copper have also been reportedwith mediocre to excellent OER catalytic activities under basicconditions.

Despite these advances, challenges for large-scale water splittingcatalysis still exist. For instance, to accomplish overall watersplitting, it is necessary to integrate both HER and OER catalysts inthe same electrolyte. Unfortunately, the current prevailing approachesoften lead to inferior overall performance because of theincompatibility of the two types of catalysts functioning under the sameconditions. Therefore, it is highly desirable to develop bifunctionaland low-cost electrocatalysts that are simultaneously active for bothHER and OER in the same electrolyte. Also, ionic conductivity is usuallyhigher at extreme pH values than under neutral conditions and theoverpotential loss of OER is much larger than that of HER, plus most OERcatalysts are vulnerable in strongly acidic media.

SUMMARY

The present disclosure in aspects and embodiments addresses thesevarious needs and problems by providing a catalyst, comprising aconductive substrate coated with a metal-phosphorus-derived film,wherein the metal is selected from the group consisting of Manganese,Iron, Cobalt, Nickel, and Copper. In some embodiments, the conductivesubstrate comprises a material selected from the group consisting ofcopper, titanium, glassy carbon, fluorine-doped tin oxide, indium-dopedtin oxide, tin, nickel, and stainless steel.

Methods for producing the catalysts and for HER and OER are alsodisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings below are supplied in order to facilitate understanding ofthe Description and Examples provided herein.

FIG. 1 is a graph showing a typical potentiodynamic deposition of acobalt-phosphorous-derived (“Co—P”) film on a copper foil substrate(scan rate: 5 mV/s).

FIG. 2(a) is an SEM image showing a Co—P film on a Cu foil substrate,with an inset image showing a cross-section of the film. The scale barsfor both the SEM image and the inset image are 5 μm.

FIG. 2(b) is a high resolution XPS spectra of the Co 2p region of a Co—Pfilm.

FIG. 2(c) is a high resolution XPS spectra of the P 2p region of a Co—Pfilm.

FIG. 3 shows an XPS survey of the as-prepared Co—P film.

FIG. 4(a) is a graph showing the polarization curves of a Co—P film(circles), a platinum-carbon-loaded electrode (“Pt—C”) (dashed) andblank Cu foil (solid line) in 1 M KOH at a scan rate of 2 mV/s androtating rate of 2,000 rpm. The inset shows the amplified region aroundthe onsets of those polarization curves.

FIG. 4(b) is a graph showing the Tafel plots corresponding to thepolarization curves shown in FIG. 4(a) for the Co—P film (solid circles)and Pt—C(open circles). The corresponding linear fittings for each plotare shown in dashed and solid lines, respectively.

FIG. 4(c) is a graph showing the long-term controlled potentialelectrolysis of a Co—P film (shown in a solid line) and blank Cu foil(shown in a dotted line) in 1 M KOH at an overpotential of 107 mV. Theinset is a graph showing the corresponding current change over time ofCo—P film (shown in a solid line) and blank Cu foil (shown in a dottedline) during the electrolysis.

FIG. 4(d) is an SEM image of a Co—P film after 2 h of H₂ evolutionelectrolysis at η=−107 mV.

FIG. 5 is an SEM image of the post-HER Co—P film.

FIG. 6(a) shows XPS spectra of the Co 2p regions of a Co—P film afterHER electrolysis (top) and after OER electrolysis (bottom).

FIG. 6(b) shows XPS spectra of the P 2p regions of a Co—P film after HERelectrolysis (top) and after OER electrolysis (bottom).

FIG. 7(a) shows a cyclic voltammogram of the as-prepared Co—P filmsbefore HER electrolysis in the non-Faradaic region.

FIG. 7(b) shows a cyclic voltammogram of the Co—P films after HERelectrolysis at η=−107 mV in 1 M KOH in the non-Faradaic region.

FIG. 7(c) shows the scan rate dependence of the current densities of theas-prepared and post-HER Co—P films at −0.90 V vs Ag/AgCl.

FIG. 8(a) is a graph showing the polarization curves of a Co—P film(circles), an iridium oxide-loaded electrode (“IrO₂”) (dashed line), andblank Cu foil (solid line) in 1 M KOH at scan rate of 2 mV/s androtating rate of 2000 rpm. The inset shows the amplified region aroundthe onsets of the polarization curves of Co—P/Cu and IrO₂.

FIG. 8(b) is a graph showing the Tafel plots corresponding to thepolarization curves shown in FIG. 4(a) for the Co—P film (circles) andIrO₂ (long dashed line). The corresponding linear fittings for each plotare shown in short dashed and solid lines, respectively.

FIG. 8(c) is a graph showing the long-term controlled potentialelectrolysis of a Co—P film (shown in a solid line) and blank Cu foil(shown in a dotted line) in 1 M KOH at an overpotential of 343 mV. Theinset is a graph showing the corresponding current change over time ofthe Co—P film (shown in a solid line) and blank Cu foil (shown in adotted line) during the electrolysis.

FIG. 8(d) is an SEM image of a Co—P film after a 2 h OER electrolysis atη=343 mV.

FIG. 9 shows an XPS survey of the post-OER Co—P film.

FIG. 10(a) is a graph showing the polarization curves for two electrode(anode/cathode) configurations, including Co—P/Co—P (circles),IrO₂/Pt—C(long dashed line), Pt—C/Pt—C(short dashed line), and IrO₂/IrO₂(solid line) configurations for overall water splitting in 1 M KOH at ascan rate of 2 mV/s. The inset shows the amplified region around theonsets of those polarization curves.

FIG. 10(b) is a graph showing the Tafel plots corresponding to thepolarization curves shown in FIG. 10(a) for the Co—P/Co—P (circles),IrO₂/Pt—C(long dashed line), Pt—C/Pt—C (short dashed line), andIrO₂/IrO₂ (solid line) configurations and their associated linearfittings.

FIG. 10(c) is a graph showing the long-term controlled potentialelectrolysis of Co—P/Co—P (solid line) and IrO₂/Pt—C(dashed line) in 1 MKOH. The inset shows the corresponding current change over time ofCo—P/Co—P (solid line) and IrO₂/Pt—C(dashed line).

FIG. 10(d) is a graph showing the generated H₂ and O₂ volumes over timeversus theoretical quantities assuming a 100% Faradaic efficiency forthe overall water splitting of Co—P/Co—P in 1 M KOH at 1=400 mV.

FIG. 11 is a graph showing a typical potentiodynamic deposition of anickel-phosphorous-derived (“Co—P”) films on a copper foil substrate(scan rate: 5 mV/s).

FIG. 12(a) is a graph showing the HER polarizations (1.0 M KOH; scanrate=2 mV/s) of Ni—P films prepared via different potentiodynamiccycles.

FIG. 12(b) is a graph showing the OER polarizations (1.0 M KOH; scanrate=2 mV/s) of Ni—P films prepared via different potentiodynamiccycles.

FIG. 13(a) is an SEM image showing a Ni—P film.

FIG. 13(b) is an SEM image of a cross section of a Ni—P film.

FIG. 13(c) is a high resolution XPS spectra of the Ni 2p region of anas-prepared Ni—P film, a post-HER Ni—P film and a post OER Ni—P film.

FIG. 13(d) is a high resolution XPS spectra of the P 2p region of anas-prepared Ni—P film, a post-HER Ni—P film and a post OER Ni—P film.

FIG. 14 shows XRD patterns of an as-prepared Ni—P film (top) and a blankcopper foil (bottom).

FIG. 15(a) is a graph showing HER polarization curves of Ni—P film, aplatinum-carbon-loaded electrode (“Pt—C”), a NiO_(x) catalyst film and ablank Cu foil in 1.0 M KOH at a scan rate of 2 mV/s and rotating rate of2000 rpm. The inset shows the amplified region around the catalyticonsets.

FIG. 15(b) is a graph showing the Tafel plots corresponding to thepolarization curves shown in FIG. 15(a) for the Ni—P film and NiO_(x).The corresponding linear fittings for each plot are shown in dashedlines.

FIG. 15(c) is a graph showing HER polarization curves for Ni—P filmbefore (solid line) and after (dashed line) 1000 continuous cyclicvoltammetric sweeps from 0 to −0.15 V vs RHE in 1.0 M KOH.

FIG. 15(d) is a representative SEM image of Ni—P film after 2 hcontrolled potential electrolysis at η=−110 mV in 1.0 M KOH.

FIG. 16 shows controlled potential electrolysis of Ni—P in 1.0 M KOH atan overpotential of −110 mV. The inset shows the corresponding currentchange over time.

FIG. 17(a) shows a cyclic voltammogram of the as-prepared Ni—P filmsbefore HER electrolysis in the non-Faradaic region.

FIG. 17(b) shows a cyclic voltammogram of the Ni—P films after HERelectrolysis at η=−110 mV in 1 M KOH in the non-Faradaic region.

FIG. 17(c) shows the scan rate dependence of the current densities ofthe as-prepared and post-HER Ni—P films at −0.85 V vs Ag/AgCl.

FIG. 18(a) is a graph showing OER polarization curves of Ni—P film,IrO₂, NiO_(x) and blank Cu film in 1.0 M KOH at a scan rate of 2 mV/sand rotating rate of 2000 rpm (the inset shows the amplified regionaround the catalytic onsets).

FIG. 18(b) is a graph showing the Tafel plots corresponding to thepolarization curves shown in FIG. 17(a) for the Ni—P film, IrO₂, andNiO_(x) with their associated linear fittings (dashed lines).

FIG. 18(c) is a graph showing OER polarization curves for Ni—P filmbefore (solid line) and after (dashed line) 1000 continuous cyclicvoltammetric sweeps from 1.50 to 1.65 V vs RHE in 1.0 M KOH.

FIG. 18(d) is a representative SEM image of Ni—P film after 2 hcontrolled potential electrolysis at q=350 mV in 1.0 M KOH.

FIG. 19 is a graph showing OER polarization curves for Ni—P film before(solid line) and after (dashed line) 1,000 continuous cyclicvoltammetric sweeps from 1.0 to 1.7 V vs RHE in 1.0 M KOH.

FIG. 20 shows the Raman spectra of the as-prepared, post-HER, andpost-OER samples of Ni—P.

FIG. 21 is an FTIR spectra of the electrolyte solution (1.0 M KOH) priorto (top) and post (bottom) OER.

FIG. 22 is a schematic showing an electrical equivalent circuit used tomodel the Ni—P catalysis system for both HER and OER.

FIG. 23(a) is a Nyquist plot of Ni—P films for HER under variousoverpotentials, where the solid lines are corresponding fitting curves.The inset shows the plot of the logarithm R_(p) versus overpotential andthe corresponding fitting curve.

FIG. 23(b) is a Nyquist plot of Ni—P films for OER under variousoverpotentials, where the solid lines are corresponding fitting curves.The inset shows the plot of the logarithm R_(p) versus overpotential andthe corresponding fitting curve.

FIG. 24 shows Bode plots of Ni—P for HER.

FIG. 25 shows Bode plots of Ni—P for OER.

FIG. 26 shows OER polarization curves (scan rate: 2 mV/s) of Ni—P in KOHof different concentrations.

FIG. 27(a) is a graph showing Tafel plots of Ni—P film for OER in 1.0 to5.0 M KOH.

FIG. 27(b) is a graph showing potentials at current density of 5 and 10mA/cm² versus the logarithm hydroxide anion activity. Dash lines are thecorresponding fitting curves.

FIG. 28 is a graph showing polarization curves (scan rate: 2 mV/s) of aNi—P/Ni—P catalyst couple for overall water splitting before (solidline) and after (dashed line) 1,000 continuous cyclic voltammetry cyclesfrom 1.5 V to 1.65 V vs RHE in 1.0 M KOH.

FIG. 29(a) is a graph showing 24 h chronopotentiometry of the Ni—P/Ni—Pcatalyst couple for overall water splitting in 1.0 M KOH with a currentdensity at 10 mA/cm².

FIG. 29(b) is a graph showing 24 h chronopotentiometry of the Ni—P/Ni—Pcatalyst couple for overall water splitting in 1.0 M KOH with potentialevolution over time.

DETAILED DESCRIPTION

The present disclosure covers apparatuses and associated methods for theproduction and related applications of metal-phosphorous-derived filmsas hydrogen evolution catalysts. In the following description, numerousspecific details are provided for a thorough understanding of specificpreferred embodiments. However, those skilled in the art will recognizethat embodiments can be practiced without one or more of the specificdetails, or with other methods, components, materials, etc. In somecases, well-known structures, materials, or operations are not shown ordescribed in detail in order to avoid obscuring aspects of the preferredembodiments. Furthermore, the described features, structures, orcharacteristics may be combined in any suitable manner in a variety ofalternative embodiments. Thus, the following more detailed descriptionof the embodiments of the present invention, as illustrated in someaspects in the drawings, is not intended to limit the scope of theinvention, but is merely representative of the various embodiments ofthe invention.

In this specification and the claims that follow, singular forms such as“a,” “an,” and “the” include plural forms unless the content clearlydictates otherwise. All ranges disclosed herein include, unlessspecifically indicated, all endpoints and intermediate values. Inaddition, “optional” or “optionally” refer, for example, to instances inwhich subsequently described circumstance may or may not occur, andinclude instances in which the circumstance occurs and instances inwhich the circumstance does not occur. The terms “one or more” and “atleast one” refer, for example, to instances in which one of thesubsequently described circumstances occurs, and to instances in whichmore than one of the subsequently described circumstances occurs.

The present disclosure covers methods, compositions, reagents, and kitsfor metal-phosphorus-derived films as competent hydrogen evolutioncatalysts or oxygen evolution catalysts.

Aspects of the present disclosure may be further described in Jian, N.,You, B, Sheng, M, and Sun, Y., Bifunctionality and Mechanism ofElectrodepoited Nickel-Phosphorus Films for Efficient Overall WaterSplitting, 8 ChemCatChem 1-6-112 (Dec. 4, 2015) and in Jian, N., You, B,Sheng, M, and Sun, Y., Electrodeposited Cobalt-Phosphorus-Derived Filmsas Competent Bifunctional Catalysts for Overall Water Splitting, 54Angew, Chem. Int. Ed. 6251-6254 (Apr. 20, 2015). The entirety of thesepapers are incorporated herein by reference.

In embodiments, the catalysts include a conductive substrate coated witha metal-phosphorus-derived film.

Conductive Substrates:

Any suitable conductive material capable of being coated with ametal-phosphorus-derived film may be employed in the catalyst. Exemplaryconductive substrates include: copper, titanium, glassy carbon,fluorine-doped tin oxide, indium-doped tin oxide, nickel, tin, andstainless steel. Metal foils, such as copper foil may be employed inembodiments where the conductive substrate is a metal.

Metal-Phosphorus-Derived Film:

The metal-phosphorus-derived films may use any suitable metal source.Exemplary metals that may be employed in the metal-phosphorus-derivedfilms include: manganese, iron, cobalt, nickel, and copper. In someembodiments, combinations of more than one metal may be used to producethe metal-phosphorus-derived film.

The metal-phosphorus-derived films may be configured to have a suitableconcentration of phosphorus. Exemplary phosphorus/metal ratios includefrom about 1/20 to about 1/1. In embodiments, phosphorus may be presentin the metal-phosphorus-derived films in concentrations, by atomicpercentage, of greater than 0 to about 50%, from about 5% to about 50%,from about 5% to about 20%, and about 10%. The ratio of phosphorus tometal may be adjusted depending on the metal being used. For example, insome embodiments employing cobalt as the metal, phosphorous may bepresent, by atomic percentage, in concentrations of about 5% to about15%, from about 7% to about 12%, or about 10%. In embodiments employingnickel as the metal, phosphorous may be present, by atomic percentage,in concentrations of about 25% to about 35%, from about 27% to about32%, or about 30%.

Any suitable metal source may be employed. In embodiments, affordablemetal sources are preferred. Metal chlorides, nitrates, and sulfatessalts may be used. For cobalt-phosphorus-derived films, exemplary cobaltsources include cobalt chloride, cobalt sulfate, and cobalt nitrate. Fornickel-phosphorus-derived films, exemplary nickel sources include nickelchloride, nickel sulfate, and nickel nitrate. For manganese, iron, andcopper-phosphorous-derived films, exemplary metal sources includemetal-chlorides, sulfates, and nitrates.

Any suitable phosphorus source may be used. In embodiments, exemplaryphosphorus sources include NaH₂PO₂.

Production Methods:

Catalysts described in this application may be produced byelectrodeposition of the metal-phosphorus-derived film on the conductivesubstrate. Potentiodynamic deposition methods may be employed to producecatalysts. For example, a NiP film may be readily prepared bypotentiodynamic deposition from NiCl₂ and NaH₂PO₂ in the presence ofglycine. In such an embodiment, glycine plays an important role incontrolling the deposition potential and rate of the Ni—P film.

Exemplary films may reach current densities of 10 mAcm⁻² withoverpotentials of −93 to −94 mV for HER and 344 to 345 mC for OER withvery small Tefel slopes of 42 to 43 and 47 to 49 mV dec⁻¹, for HER andOER respectively.

Electrolysis Solutions:

Any suitable electrolysis solution may be used for the production ofhydrogen or oxygen. Suitable electrolysis solutions include aqueoussolutions comprising a conductive electrolyte. In some embodiments, analkaline electrolyte or combination thereof, such as KOH or NaOH, may beused in about 1.0 M concentrations. In some embodiments, theelectrolysis solution has a pH of from about 7 to about 14, or about 14.

EXAMPLES

The following examples are illustrative only and are not intended tolimit the disclosure in any way.

Example 1 Cobalt-Phosphorous-Derived Films

As described in detail below, cobalt-phosphorous-derived (“Co—P”) filmswere deposited onto a copper foil substrate using a facilepotentiodynamic electrodeposition with cobalt and phosphorous reagents.The as-prepared Co—P films can be directly utilized as electrocatalystsfor both HER and OER in strong alkaline electrolyte, which can achieve acurrent density of 10 mA/cm² with overpotentials of −94 mV for HER and345 mV for OER with very small Tafel slopes, 45 and 47 mV/dec,respectively. When the Co—P films were deposited on an anode and cathodefor overall water splitting, the superior activity and stability of thecatalytic films can even compete versus the integrated Pt and IrO₂catalyst couple.

Materials

Cobalt sulfate, sodium acetate, sodium hypophosphite monohydrate,potassium hydroxide were purchased from commercial vendors and used asreceived. Pt—C (20% Pt on Vulcan XC-72) and iridium (IV) oxide werepurchased from Premeteck Co. and Alfa Aesa, respectively, and used asreceived. Nafion 117 solution (5% in a mixture of lower aliphaticalcohols and water) was purchased from Sigma-Aldrich. Copper foils (3M™copper conductive tapes, single adhesive surface) were purchased fromTed Pella, Inc. Water was deionized (18Ω) with a Barnstead E-Puresystem.

Electrochemical Methods

Electrochemical experiments were performed on Gamry Interface 1000potentiostats. Aqueous Ag/AgCl reference electrodes (saturated KCl) werepurchased from CH Instruments. The reference electrode in aqueous mediawas calibrated with ferrocenecarboxylic acid whose Fe^(3+/2+) couple is0.284 V vs SCE. All potentials reported in this paper were convertedfrom vs Ag/AgCl to vs RHE by adding a value of 0.197+0.059×pH to vs RHE.iR (current times internal resistance) compensation was applied inpolarization and controlled potential electrolysis experiments toaccount for the voltage drop between the reference and workingelectrodes using the Gamary Framework™ Data Acquisition Software 6.11.

Preparation of Co—P Films

Prior to electrodeposition, copper foils were rinsed with water andethanol thoroughly to remove residual organic species. For linear sweepvoltammetry experiments, a circular copper foil with a 3 mm diameter wasprepared and pasted on the rotating disk glassy carbon electrode, thenthe assembled electrode was exposed to the deposition solution (50 mMCoSO₄, 0.5 M NaH₂PO₂, and 0.1 M NaOAc in water). A platinum wire wasused as the counter electrode and a Ag/AgCl (sat. KCl) electrode as thereference electrode. Nitrogen was bubbled through the electrolytesolution for at least 20 min prior to deposition and maintained duringthe entire deposition process. The potential of consecutive linear scanswas cycled 15 times between −0.3 and −1.0 V vs Ag/AgCl at a scan rate of5 mV/s under stirring and a rotation rate of 500 rpm. After deposition,the assembled electrode was removed from the deposition bath and rinsedwith copious water gently. The prepared Co—P film can be directly usedto collect its polarization curves or stored under vacuum at roomtemperature for future use. For samples prepared for controlledpotential electrolysis, a copper foil was directly used the workingelectrode with a geometric area of 0.3 cm² exposed to the electrolyte.The deposition potential window and cycle number are the same asaforementioned. Typical potentiodynamic depositions of Co—P films areshown in FIG. 1.

Preparation of Pt—C and IrO₂-Loaded Electrodes

12 mg Pt—C or IrO₂ were dispersed in a 2 mL mixture solution containing800 μL water, 120 μL 5% Nafion solution, and 1.08 mL ethanol, followedby sonication for 30 min to obtain a homogeneous catalyst ink. 2 μLcatalyst ink was loaded on the surface of a glassy carbon electrode(surface area: 0.07065 cm²) for 6 times. Consequently, the overallloading amount is 1 mg/cm².

Physical Methods

Scanning electron microscopy images and elemental mapping analysis werecollected on a FEI QUANTA FEG 650 (FEI, USA) by FenAnn Shen at theMicroscopy Core Facility of USU. Cobalt and phosphorous analysis wereobtained on a Thermo Electron iCAP inductively coupled plasmaspectrophotometer at the Analytical Laboratory of USU. X-rayphotoelectron spectroscopy analyses were done using a Kratos Axis Ultrainstrument (Chestnut Ridge, N.Y.) at Surface Analysis and NanoscaleImaging group at the University of Utah, sponsored by the College ofEngineering, Health Sciences Center, Office of the Vice President forResearch, and the Utah Science Technology and Research (USTAR)Initiative of the State of Utah. The samples were affixed on a stainlesssteel Kratos sample bar, loaded into the instrument's load lock chamber,and evacuated to 5×10⁻⁸ torr before it was transferred into the sampleanalysis chamber under ultrahigh vacuum conditions (˜10⁻¹⁰ torr). X-rayphotoelectron spectra were taken using the monochromatic Al Kα source(1486.7 eV) at a 300×700 μm spot size. Low resolution survey and highresolution region scans at the binding energy of interest were taken foreach sample. To minimize charging, samples were flooded with low-energyelectrons and ions from the instrument's built-in charge neutralizer.The samples were also sputter cleaned inside the analysis chamber with 1keV Ar⁺ ions for 30 seconds to remove adventitious contaminants andsurface oxides. Data were analyzed using CASA XPS software, and energycorrections on high resolution scans were done by referencing the C1speak of adventitious carbon to 284.5 eV. This work also made use of theSurface Analysis and Nanoscale Imaging group at the University of Utah.The generated hydrogen volume during electrolysis was quantified with aSRI gas chromatography system 8610C equipped with a Molecular Sieve 13×packed column, a HayesSep D packed column, and a thermal conductivitydetector. The oven temperature was maintained at 60° C. and argon wasused as the carrier gas.

Results

Cobalt-phosphorous-derived (“Co—P”) films were deposited onto a copperfoil substrate using a facile potentiodynamic electrodeposition withcobalt and phosphorous reagents. FIG. 1 shows a typical potentiodynamicdeposition of Co—P films on a copper foil substrate (scan rate: 5 mV/s).Scanning electron microscopy (SEM) images of the as-prepared Co—P filmshow nearly complete coverage of the rough film on copper foil (FIG.2(a)), with no observed crystalline particles or aggregates. Elementalmapping analysis indicated Co and P were distributed evenly over theentire film (data not shown). The cross section SEM image reveals thethickness of the film is about 1-3 μM (FIG. 2(a) inset). The X-rayphotoelectron spectroscopy (XPS) survey of the as-prepared film (FIG. 3)shows all the anticipated elements. The Co 2p XPS spectrum (FIG. 2(b))displays two peaks at 778.3 and 793.4 eV, corresponding to the Co2p_(3/2) and 2p_(1/2) binding energies, respectively. These values areextremely close to those of metallic cobalt. The P 2p XPS spectrum (FIG.2(c)) exhibits a dominant peak at 129.5 eV, which can be attributed tothe phosphide signal. A broad feature at ˜133.6 eV is assigned tophosphate. In addition, as shown in Table 1 below, elemental analysis ofthe as-prepared Co—P film measured the amount of Co and P as 2.52 and0.19 mg/cm², respectively, with a molar ratio of about 6.98.

TABLE 1 ICP-OES data of as-prepared Co—P film, and of Co—P film after 2h HER electrolysis and after 2 h OER electrolysis. Area [Co] [P] Co/Pmole Sample (cm²) (mg/cm²) (mg/cm²) ratio Fresh prepared 0.3 2.52 0.196.98 After 2 h HER electrolysis 0.3 2.48 0.12 10.5 After 2 h OERelectrolysis 0.3 2.47 0.13 9.74

We first evaluated the HER activity of a Co—P film in strong alkalinesolution (See FIGS. 4(a)-4(d)), by comparing the performance of the Co—Pfilm to a platinum-carbon-loaded electrode (“Pt—C”) and blank Cu foil.The blank copper foil did not show any HER catalytic activity before−0.3 V vs a reversible hydrogen electrode (“RHE”) (See FIG. 4(a)). Incontrast, a rapid cathodic current rise was observed for the Co—P filmbeyond −50 mV vs RHE (FIG. 4(a) inset). Further scanning towardsnegative potential produced a dramatic increase in current density alongwith vigorous evolution of H₂ bubbles from the electrode surface. TheCo—P film required an overpotential (η) of only −94 mV to reach acurrent density of 10 mA/cm². As shown in Table 2, such a lowoverpotential requirement compares favorably to other reported HERcatalysts at pH 14:

TABLE 2 Comparison of selected nonprecious HER electrocatalysts inalkaline media. Tafel j slop (mA η (mV Catalysts Electrolyte cm⁻²) (mV)dec⁻¹) Reference Co—P film 1M 10 94  42 This work KOH 20 115 100 158CoP/CC 1M 10 209 129 J. Am. Chem. Soc. KOH 100 >500 2014, 136, 7587Co—S/FTO 1M 1 480 N/A J. Am. Chem. Soc. KOH 2013, 135, 17699 Co— 1M 10370 N/A Angew. Chem. Int. Ed. NRCNTs KOH 20 >450 2014, 53, 4372. Ni₂P 1M20 205 N/A J. Am. Chem. Soc. KOH 2013, 135, 9267. Ni/Ni(OH)₂ 0.1M10 >300 128 Angew. Chem. Int. Ed. KOH 2012, 51, 12495. MoB 0.1M 10 225 59 Angew. Chem. Int. Ed. KOH 2012, 51, 12703. MoS_(2+x)/FTO 1M 10 310N/A Angew. Chem. Int. Ed. KOH 2015, 54, 667. Amorphous 0.1M 10 540 N/AChem. Sci. MoS_(x) KOH 2011, 2, 1262. FeP 1M 10 218 146 ACS Catal.NAs/CC KOH 2014, 4, 4065.

Remarkably, the Co—P film was able to produce a catalytic currentdensity of 1000 mA/cm² within an overpotential of −227 mV. The linearfitting of its Tafel plot (FIG. 4(b)) rendered a Tafel slope of 42mV/dec, which is among the smallest Tafel slopes of reported HERcatalysts in alkaline media (See Table 2). Although Pt—C exhibited avery small catalytic onset potential, its Tafel slope (108 mV/dec) wassignificantly larger than that of the Co—P film (FIG. 4(b)). Therefore,beyond −167 mV vs RHE, the catalytic current density of Co—P surpassedthat of Pt—C. Additionally, the Co—P film also exhibited superiorlong-term stability. A 24 h controlled potential electrolysis at η=−107mV showed a nearly linear charge accumulation and steady current overthe entire course of electrolysis (FIG. 4(c)). The blank copper foilonly generated negligible charge build-up under the same condition.

To probe the morphology and composition of the Co—P film after HERelectrocatalysis, the SEM and XPS results of a post-HER Co—P film werecollected. As shown in FIG. 4(d), the film still maintained a uniformcoverage on the copper foil and no apparent clusters or aggregates wereobserved (FIG. 5). Elemental mapping analysis confirmed the evendistribution of Co and P in the post-HER film (data not shown). The Co2p XPS spectrum of the Co—P film after HER electrolysis shows two peaksat 793.2 and 778.2 eV (FIG. 6(a), top), corresponding to Co 2p_(3/2) and2p_(1/2) states, respectively. The similarity of the Co 2p peaks of thepost-HER Co—P film compared to those of the as-prepared Co—P film (FIG.2b ) implies the major composition of the film preserved as metalliccobalt during HER. Further, a peak at 129.3 eV was observed from the P2p XPS spectrum of the post-HER sample (FIG. 6(b), top); while thephosphate peak at 133.6 eV originally observed for the as-prepared Co—Pfilm (FIG. 2(c)) was absent. Its absence is likely due to thedissolution of cobalt phosphate under cathodic condition. As shown inFIG. 7(a)-7(c), the as-prepared and post-HER Co—P films exhibitedsimilar capacitance, implying their similar electrical active surfacearea. Elemental analysis of the post-HER film resulted in Co and Pamount of 2.48 and 0.12 mg/cm² with a Co/P ratio of 10.5 (See Table 1above).

We next assessed the catalytic activity of the Co—P film for OER in thesame electrolyte (See FIGS. 8(a)-8(d)), by comparing the performance ofthe Co—P film to an iridium oxide-loaded electrode (“IrO₂”) and blank Cufoil (“Blank”). As expected, a blank copper foil did not showappreciable anodic current before 1.7 V vs RHE (FIG. 8(a)). The OERcatalytic current density of the Co—P film increased dramatically beyond1.53 V vs RHE (FIG. 8a , inset). It could reach current densities of 10,100, and 500 mA/cm² at η=345, 413, and 463 mV, respectively, lower thanthose of IrO₂ and many other reported OER catalysts, as shown in Table3:

TABLE 3 Comparison of selected nonprecious OER electrocatalysts inalkaline media. η Tafel (mV) at slop 10 mA (mV Catalysts Electrolytecm⁻² dec⁻¹) Reference Co—P film 1.0M 345 47 This work KOH NiCo LDH 1.0M367 40 Nano Lett. KOH 2015, 15, 1421. Cu—N—C/ 0.1M >770 N/A Nat. Commun.graphene KOH 2014, 5, 5285. CoCo LDH 1.0M 393 59 Nat. Commun. KOH 2014,5, 4477. Co₃O₄/rm-GO 1.0M 310 67 Nat. Mater. KOH 2011, 10, 780.MnO_(x)/Au 0.1M >480 N/A J. Am. Chem. Soc. KOH 2014, 136, 4920.Ca₂Mn₂O₅/C 0.1M >470 149  J. Am. Chem. Soc. KOH 2014, 136, 14646.Co_(x)O_(y)/NC 0.1M 430 N/A Angew. Chem. Int. Ed. KOH 2014, 53, 8508.De-LiCoO₂ 0.1M >400 50 Nat. Commun. KOH 2014, 5, 4345. CoMn LDH 1.0M 32443 J. Am. Chem. Soc. KOH 2014, 136, 16481. NiFeOx film 1.0M >350 N/A J.Am. Chem. Soc. NaOH 2013, 135, 16977. CoO/NG 1.0M 340 71 Energy Environ.Sci. KOH 2014, 7, 609. CoO_(x) film 1.0M 403 42 J. Am. Chem. Soc. KOH2012, 134, 17253. α-MnO₂—SF 0.1M 490   77.5 J. Am. Chem. Soc. KOH 2014,136, 11452 MnO_(x) film 1.0M 563 49 J. Am. Chem. Soc. KOH 2012, 134,17253. NiFeO_(x) film 1.0M >350 N/A J. Am. Chem. Soc. NaOH 2013, 135,16977. Fe—Ni oxides 1.0M >375 51 ACS Catal. KOH 2012, 2, 1793.Zn_(x)Co_(3-x)O₄ 1.0M 330 51 Chem. Mater. nanowire KOH 2014, 26, 1889.Ni_(x)Co_(3-x)O₄ 1.0M ~370 59-64 Adv. Mater. nanowire KOH 2010, 22,1926.Linear fitting of its Tafel plot resulted in a Tafel slope of 47 mV/dec(FIG. 8(b). As one of the state-of-the-art OER catalysts, IrO₂ was ableto catalyze OER at a lower onset of ˜1.50 V vs RHE. However, itsperformance was quickly exceeded by that of the Co—P film beyond 1.58 Vvs RHE. In fact, the Tafel slope of Co—P (47 mV/dec) is even lower thanthat of IrO₂ (55 mV/dec), demonstrating more favorable OER kinetics ofthe former. Besides high OER activity, the Co—P film also featuresexcellent stability, as revealed by a 24 h controlled potentialelectrolysis at η=343 mV (FIG. 8(c)).

The SEM image of the post-OER Co—P film (FIG. 8(d)) indicates itcontains large nanoparticle aggregates, in sharp contrast to the roughand porous morphology of the as-prepared and post-HER samples.Nevertheless, elemental mapping analysis still demonstrated an evendistribution of Co and P in the film (data not shown), plus a largeconcentration of O. Indeed, an intense O is peak was observed in the XPSsurvey spectrum of the post-OER film (FIG. 9). The Co 2p spectrumdisplayed two peaks at 780.7 and 796.3 eV (FIG. 6(a), bottom), which canbe assigned to oxidized cobalt, Co₃O₄, plus its satellite peaks at 786.3and 802.7 eV. However, the metallic cobalt 2p peaks at 778.0 and 793.0eV could still be well resolved. The P 2p spectrum showed a phosphatepeak at 133.2 eV (FIG. 6(b)), whereas the original phosphide feature at129.5 eV disappeared completely. Taken together, it indicates that theoriginal cobalt in the Co—P film was partially oxidized to Co₃O₄ andcobalt phosphate during OER. An OER electrocatalyst with a metalliccobalt core and cobalt oxide/hydroxide shell was reported. Elementalanalysis of the post-OER film resulted in the remaining amount of Co andP as 2.47 and 0.13 mg/cm² with a Co/P ratio of 9.74 (See Table 1), stillsimilar to those of the post-HER film.

Based on the results aforementioned, we anticipated that the Co—P filmcould act as a bifunctional electrocatalyst for overall water splitting.Hence, a two-electrode configuration was employed, with Co—P and on Cusubstrates were used as both the anode and the cathode (i.e.,Co—P/Co—P). The performance of the Co—P/Co—P configurations werecompared to the performance of IrO₂/Pt—C, Pt—C/Pt—C, and IrO₂/IrO₂configurations (see FIGS. 10(a)-10(d)). When the as-prepared Co—P filmswere used as electrocatalysts for both anode and cathode (Co—P/Co—Pcouple), a catalytic current was observed when the applied potential waslarger than 1.56 V with a Tafel slope of 69 mV/dec.

The rapid catalytic current density exceeded 100 mA/cm² at 1.744 V. WhenPt—C or IrO₂ was used for both electrodes (Pt—C/Pt—C or IrO₂/IrO₂couple), much diminished catalytic current densities were obtained withlarge Tafel slopes of 166 and 290 mV/dec, respectively. Since Pt iswell-known for HER and IrO₂ for OER, the integration of Pt—C on cathodeand IrO₂ on anode was expected to produce an excellent catalytic system.Indeed, the IrO₂/Pt—C couple was able to catalyze water splitting withan onset around 1.47 V (FIG. 10(a), inset). However, the Tafel slope ofIrO₂/Pt—C is 91 mV/dec, larger than that of Co—P/Co—P (69 mV/dec).Therefore, when the applied potential was higher than 1.67 V, Co—P/Co—Pwas able to surpass IrO₂/Pt—C in catalyzing overall water splitting. Inaddition, the Co—P/Co—P couple maintained excellent stability asmanifested by the steady current change and nearly linear chargeaccumulation for a 24 h electrolysis (FIG. 10(c)). In fact, theintegrated activity of IrO₂/Pt—C was slightly inferior to that of theCo—P/Co—P couple under the same condition. FIG. 10(d) shows the producedH₂ and O₂ quantified by gas chromatography match the calculated amountbased on passed charge well and the volume ratio of H₂ and O₂ is closeto 2, leading to a Faradaic efficiency of 100%.

In conclusion, we have reported electrodeposited Co—P films can act asbifunctional catalysts for overall water splitting. The catalyticactivity of the Co—P films can rival the state-of-the-art catalysts,requiring η=−94 mV for HER and η=345 mV for OER to reach 10 mA/cm² withTafel slopes of 45 and 47 mV/dec, respectively. It can be directlyutilized as catalysts for both anode and cathode with superiorefficiency, strong robustness, and 100% Faradaic yield. Theunderstanding of real-time composition and structural evolution of thefilm during electrolysis requires in situ spectroscopic study, which isunder current investigation.

Example 2 Nickel-Phosphorous-Derived Films

As described in detail below, nickel-phosphorous-derived (“Ni—P”) filmswere deposited onto a copper foil substrate using a facilepotentiodynamic electrodeposition with cobalt and phosphorous reagents.The as-prepared Ni—P films can be directly utilized as electrocatalystsfor both HER and OER in strong alkaline electrolyte, (1.0 M KOH).

Materials

Nickel chloride hexahydrate (NiCl₂.6H₂O), glycine, sodium hypophosphitemonohydrate (NaH₂PO₂—H₂O), sodium acetate (NaOAc), and potassiumhydroxide (KOH) were all purchased from commercial vendors and useddirectly without any further purification. Nafion 117 solution (5% in amixture of lower aliphatic alcohols and water) was purchased fromSigma-Aldrich. Copper foils (3M™ copper conductive tapes, singleadhesive surface) were purchased from Ted Pella, Inc. Water wasdeionized (18 MQ) using a Barnstead E-Pure system.

Preparation of Catalyst Films (Ni—P and NiO_(x))

Prior to electrodeposition, copper foils were rinsed with water andethanol thoroughly to remove residual organic species. For linear sweepvoltammetry experiments, a circular copper foil with a 3 mm diameter wasprepared and pasted on the rotating disk glassy carbon electrode, andthen the assembled electrode was exposed to the optimized depositionsolution (50 mM NiCl₂, 1 M NaH₂PO₂, 0.16 M glycine, and 0.1 M NaOAc inwater). A platinum wire was used as the counter electrode and a Ag/AgCl(sat. KCl) electrode as the reference electrode. Nitrogen was bubbledthrough the electrolyte solution for at least 20 min prior to depositionand maintained during the entire deposition process. The potential ofconsecutive linear scans was cycled between 0.1 and −1.1 V vs Ag/AgCl ata scan rate of 5 mV/s and a rotation rate of 500 rpm (FIG. 11). Afterdeposition, the working electrode was rinsed with water and acetonegently and dried under vacuum at room temperature, followed by directuse for electrocatalysis.

The HER and OER polarization curves of Ni—P films prepared viapotentiodynamic deposition cycles of 5, 10, 15, and 20 are compared inFIGS. 12(a) and 12(b). It is apparent that the cycle number of 15produced a Ni—P film with the best HER and OER electrocatalyticactivities. Hence, all the electrochemical experiments discussed in themain text were conducted using the Ni—P films prepared via 15 cycles.

The control NiO_(x) catalyst films were prepared according to a reportedmethod (J. Phy. Chem. C 2014, 118, 4578-4584, the complete disclosure ofwhich is herein incorporated by reference in its entirety). Briefly, acopper foil with an exposed area of 0.3 cm² was used as the workingelectrode with platinum wire and Ag/AgCl (sat. KCl) as the counter andreference electrodes, respectively. 10 mL 0.1 M NaBi with 1.0 mMNi(NO₃)₂ was used as the electrolyte. Prior to electrodeposition, thecopper foil was rinsed with acetone and deionized water thoroughly.Electrolysis was carried out at −1.2 V vs Ag/AgCl for three hours underdeaerated condition.

Preparation of Pt—C and IrO₂-Loaded Electrodes

12 mg Pt—C or IrO₂ was dispersed in a 2 mL mixture solution containing800 μL water, 120 μL 5% Nafion solution, and 1.08 mL ethanol, followedby sonication for 30 min to obtain a homogeneous catalyst ink. 3 μLcatalyst ink was loaded on the surface of a glassy carbon electrode(surface area: 0.07065 cm²) for 6 times. Consequently, the overallloading amount is 1.53 mg/cm².

Catalyst Characterization

Powder X-ray diffractions were recorded on a Rigaku MiniflexII DesktopX-ray diffractometer. Scanning electron microscopy (SEM) images werecollected on a FEI QUANTA FEG 650 (FEI, USA). Elemental analysis ofnickel and sulfur was obtained on a Thermo Electron iCAP inductivelycoupled plasma spectrophotometer. Fourier transform infrared (FTIR)spectroscopy was conducted on an IR100 Spectrometer (Thermo Nicolet).The Raman spectra were recorded with a confocal Raman microspectrometer(Renishaw, U.K.) under a 785 nm diode laser excitation. The detection ofthe Raman signal was carried out with a Peltier cooled charge-coupleddevice (CCD) camera. The software package WIRE 3.0 (Renishaw) wasemployed for spectral acquisition and analysis. X-ray photoelectronspectroscopy analyses were conducted on a Kratos Axis Ultra instrument(Chestnut Ridge, N.Y.). The samples were affixed on a stainless steelKratos sample bar, loaded into the instrument's load lock chamber, andevacuated to 5×10⁻⁸ torr before it was transferred into the sampleanalysis chamber under ultrahigh vacuum conditions (˜10⁻¹⁰ torr). X-rayphotoelectron spectra were taken using the monochromatic Al Kα source(1486.7 eV) at a 300×700 μm spot size. High resolution region scans atthe binding energies of interest were taken for each sample. To minimizecharging, samples were flooded with low-energy electrons and ions fromthe instrument's built-in charge neutralizer. The samples were firstsputter cleaned inside the analysis chamber with 1 keV Ar⁺ ions for 30seconds to remove adventitious contaminants and surface oxides. Datawere analyzed using CasaXPS software, and energy corrections on highresolution scans were calibrated by referencing the C1s peak ofadventitious carbon to 284.5 eV.

Electrochemical Measurements

Electrochemical experiments were performed on a Gamry Interface 1000potentiostat workstation with a three-electrode cell system. Theas-prepared Ni—P (d=3 mm, S=0.07065 cm²) was used as the workingelectrode, a Ag/AgCl (sat. KCl) electrode (CH Instruments) as thereference electrode, and a platinum wire as the counter electrode. Allpotentials reported in the paper were converted to vs RHE (reversiblehydrogen electrode) by adding a value of 0.197+0.059×pH to vs RHE. iR(current times internal resistance) correction was applied for linearsweep voltammetry and controlled potential electrolysis experiments toaccount for the voltage drop between the reference and workingelectrodes using Gamry Framework™ Data Acquisition Software 6.11. Thelinear sweep voltammetry experiments were conducted in N₂ saturated 1.0M KOH electrolyte at a scan rate of 2 mV/s and a rotating speed of 2000rpm. Electric impedance spectroscopy measurements in deaerated 1.0 M KOHwere carried out in the same configuration at multiple potentials from10⁵ to 0.1 Hz with an AC potential amplitude of 30 mV.

Results

The Ni—P film can be readily prepared via potentiodynamic depositionfrom NiCl₂ and NaH₂PO₂ in the presence of glycine (FIG. 11). It is notedthat glycine plays an important role in controlling the depositionpotential and rate of the Ni—P film. The Ni—P films are able to reach acurrent density of 10 mA/cm² with overpotentials of −93 mV for HER and344 mV for OER with very small Tafel slopes of 43 and 49 mV/dec,respectively, rivaling the performance of the state-of-the-art HER andOER catalysts, Pt and IrO₂, respectively. Mechanistic studies revealedthat the catalytic rate of OER was first order in the activity ofhydroxide anion. Even more appealing is that when the Ni—P films aredeposited on the anode and cathode for overall water splitting,excellent activity and stability for overall water splitting can beachieved. Overall, the low-cost, efficient, robust, and bifunctionalnature of the Ni—P film renders it a competent catalyst for overallwater splitting.

The scanning electron microscopy (SEM) image of an as-prepared Ni—P filmis shown in FIG. 13(a), exhibiting nearly complete coverage of the roughfilm on a copper foil. The black holes in the film are potentially dueto the formation of H₂ bubbles during the deposition of the film undernegative potentials. The cross section SEM image (FIG. 13(b)) revealsthe thickness of the film is around 3 μm. The elemental mapping resultsof Ni—P confirm the presence of nickel and phosphorous which arehomogeneously distributed over the entire film (data not shown). Itspowder X-ray diffraction (XRD) pattern is almost identical to that of ablank copper foil with no unique feature that can be attributed to theNi—P film (FIG. 14). Therefore, the Ni—P film is amorphous in nature. Wefurther conducted X-ray photoelectron spectroscopy (XPS) to probe thevalence states of nickel and phosphorous in the film. As shown in FIG.13(c), the high-resolution Ni 2p XPS spectrum displays two peaks at853.0 and 870.3 eV, corresponding to the Ni 2p_(3/2) and 2p_(1/2)binding energies, respectively. These value are quite close to those ofmetallic nickel. The high-resolution P 2p spectrum (FIG. 13(d)) exhibitsa dominant feature in the region of 129 to 131 eV, which can be assignedto the anticipated phosphide signal. Elemental analysis via inductivelycoupled plasma optical emission spectrometry of the as-prepared Ni—Psamples implied the deposited amounts of nickel and phosphorous as 1.30and 0.35 mg/cm², resulting in a Ni/P atomic ratio close to 2.

The electrocatalytic activity of Ni—P was first evaluated for H₂evolution in a strongly alkaline electrolyte (1.0 M KOH), as shown inFIGS. 15(a)-15(d). A blank copper foil, commercially available 20 wt %Pt—C(Pt—C), and nickel oxide (NiO_(x)) electrodeposited on copper foilfollowing a reported method (C. He, X. Wu, Z. He, J. Phys. Chem. C 2014,118, 4578-4584) were also included as comparisons. The blank copper foildid not exhibit appreciable HER activity, with nearly no catalyticcurrent prior to −0.3 V vs RHE (reversible hydrogen electrode). On theother hand, it is anticipated that Pt—C was very active for HER. Indeed,the Pt—C catalyst exhibited a catalytic onset at nearly zerooverpotential with a quick catalytic current increase along cathodicpotential scanning. It was pleasant to see that a catalytic currentrapidly rose for Ni—P when the potential was scanned more negative than−50 mV vs RHE (FIG. 15(a), inset). Vibrant H₂ bubble growth and releasefrom the Ni—P film surface were observed upon further cathodic sweep. Anoverpotential of merely −93 mV was required for Ni—P to achieve acurrent density of 10 mA/cm², which compares favorably to many otherreported HER catalysts under strongly alkaline conditions. A detailedcomparison is listed in Table 4:

TABLE 4 Comparison of selected nonprecious HER electrocatalysts inalkaline media Tafel j slope (mA η (mV Catalysts Electrolyte cm⁻²) (mV)dec⁻¹) Reference Ni—P film 1M 10 93 43 This work KOH 20 110 100 160 Ni₂P1M 20 205 N/A J. Am. Chem. Soc. KOH 2013, 135, 9267. Ni/Ni(OH)₂ 0.1M10 >300 128  Angew. Chem. Int. Ed. KOH 2012, 51, 12495. Co—P film 1M 1094 42 Angew. Chem. Int. Ed. KOH 20 115 2015, 54, 6251. 100 158 CoP/CC 1M10 209 129  J. Am. Chem. Soc. KOH 100 >500 2014, 136, 7587. Co—S/FTO 1M1 480 N/A J. Am. Chem. Soc. KOH 2013, 135, 17699. Co— 1M 10 370 N/AAngew. Chem. Int. Ed. NRCNTs KOH 20 >450 2014, 53, 4372. MoB 0.1M 10 22559 Angew. Chem. Int. Ed. KOH 2012, 51, 12703. MoS_(2+x)/FTO 1M 10 310N/A Angew. Chem. Int. Ed. KOH 2015, 54, 667. Amorphous 0.1M 10 540 N/AChem. Sci. MoS_(x) KOH 2011, 2, 1262. FeP 1M 10 218 146  ACS Catal.NAs/CC KOH 2014, 4, 4065.

Even more remarkably, the Ni—P film was able to produce a catalyticcurrent density of 500 mA/cm² within an overpotential of −219 mV. Thederived Tafel plot (FIG. 15(b)) clearly presents two kinetic regions.Linear fittings at the low and high overpotentials rendered Tafel slopesof 43 and 81 mV/dec, respectively. It is known that water dissociationmight play a critical role under strongly alkaline conditions for H₂evolution, especially when the proton supply is insufficient at highoverpotentials. (See R. Subbaraman, D. Tripkovic, D. Strmcnik, K.-C.Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N. M. Markovic,Science 2011, 334, 1256-1260; and N. Danilovic, R. Subbaraman, D.Strmcnik, K.-C. Chang, A. P. Paulikas, V. R. Stamenkovic, N. M.Markovic, Angew. Chem. Int. Ed. 2012, 51, 12495-12498; Angew. Chem.2012, 124, 12663-12666.) Therefore, the Tafel slope obtained at the lowoverpotential region (43 mV/dec) more likely represents the intrinsicHER catalytic activity of Ni—P. Such a small Tafel slope is among thesmallest of reported HER catalysts in alkaline media (Table 4). Eventhough the onset potential of Ni—P is more negative than that of Pt—C,its Tafel slope is smaller than that of Pt—C (108 mV/dec), therefore itis anticipated that the catalytic current of Ni—P would surpass that ofPt—C at high overpotentials. In contrast, the HER activity of NiO_(x) isnegligible under this condition, slowly showing catalytic current beyond−0.2 V vs RHE. Besides high efficiency for HER, the Ni—P film alsoexhibited excellent long-term stability. FIG. 15(c) shows thepolarization curves of a Ni—P film prior to and post 1000 continuouscyclic voltammetric sweeps between 0 and −0.25 V vs RHE, whose perfectoverlap confirmed the robustness of Ni—P for extended HER catalysis.

We next collected the SEM and XPS results of a post-HER Ni—P sample todetermine possible morphology and composition change after a 2-h HERelectrocatalysis at −110 mV vs RHE when stable catalytic current wasachieved (FIG. 16). Its SEM image shown in FIG. 15(d) demonstrates thatthe post-HER film still maintained a uniform coverage on the copper foiland no apparent clusters or aggregates were observed. It is interestingto find that most of the original black holes disappeared, implying somecatalyst rearrangement took place during HER electrocatalysis. Elementalmapping analysis further confirmed the even distribution of nickel andphosphorous in the post-HER film (data not shown). FIG. 13(c) alsoincludes the high-resolution Ni 2p XPS spectrum of the post-HER film,exhibiting two peaks at 853.1 and 870.3 eV, corresponding to Ni 2p_(3/2)and 2p_(1/2) binding energies, respectively. Similarly, FIG. 13(d)displays its high-resolution P 2p spectrum showing a broad peak in128-132 eV. In fact, these XPS features are very similar to those of theas-prepared Ni—P film, indicating the overall valence states of nickeland phosphorous did not evolve substantially during HERelectrocatalysis. Moreover, the as-prepared and post-HER films exhibitedsimilar capacitance, as shown in FIGS. 17(a)-17(c), suggesting theircomparable electrochemically active surface area. Nevertheless,elemental analysis of the post-HER film resulted in nickel andphosphorous amounts of 1.06 and 0.21 mg/cm² with a Ni/P atomic ratio of2.65. These values are smaller than the original mass of the as-preparedNi—P, indicating some catalyst dissolution occurred during HERelectrocatalysis.

The OER electrocatalytic performance of Ni—P was next assessed in thesame electrolyte, 1.0 M KOH. FIG. 18(a) compares the polarization curvesof Ni—P, commercially available IrO₂, electrodeposited NiO_(x), and ablank copper foil. As expected, the blank copper foil is not an activeOER catalyst, producing negligible anodic current before 1.7 V vs RHE.In contrast, the OER catalytic current of the Ni—P film increaseddramatically beyond 1.50 V vs RHE (FIG. 18(a), inset), even comparableto the onset of IrO₂. The Ni—P film was able to produce catalyticcurrent densities of 10, 100, and 500 mA/cm² at η=344, 399, and 460 mV,respectively, lower than those of IrO₂ and many other reported OERcatalysts, as shown in Table 5:

TABLE 5 Comparison of selected nonprecious OER electrocatalysts inalkaline media. η Tafel (mV) at slope 10 mA (mV Catalysts Electrolytecm⁻² dec⁻¹) Reference Ni—P film 1.0M 344 49 This work KOH Ni₂P nanowires1.0M 400 60 Chem. Commun. KOH 2015, 51, 11626. Ni₂P 1.0M 290-330 47-59Energy Environ. Sci., KOH 2015, 8, 2347. Co—P film 1.0M 345 47 Angew.Chem. Int. Ed. KOH 2015, 54, 6251. NiCo LDH 1.0M 367 40 Nano Lett. KOH2015, 15, 1421. Cu—N—C/ 0.1M >770 N/A Nat. Commun. graphene KOH 2014, 5,5285. CoCo LDH 1.0M 393 59 Nat. Commun. KOH 2014, 5, 4477. Co₃O₄/rm-GO1.0M 310 67 Nat. Mater. KOH 2011, 10, 780. MnO_(x)/Au 0.1M >480 N/A J.Am. Chem. Soc. KOH 2014, 136, 4920. Ca₂Mn₂O₅/C 0.1M >470 149  J. Am.Chem. Soc. KOH 2014, 136, 14646. Co_(x)O_(y)/NC 0.1M 430 N/A Angew.Chem. Int. Ed. KOH 2014, 53, 8508. De-LiCoO₂ 0.1M >400 50 Nat. Commun.KOH 2014, 5, 4345. CoMn LDH 1.0M 324 43 J. Am. Chem. Soc. KOH 2014, 136,16481. NiFeOx film 1.0M >350 N/A J. Am. Chem. Soc. NaOH 2013, 135,16977. CoO/NG 1.0M 340 71 Energy Environ. Sci. KOH 2014, 7, 609. CoO_(x)film 1.0M 403 42 J. Am. Chem. Soc. KOH 2012, 134, 17253. α-MnO₂—SF 0.1M490   77.5 J. Am. Chem. Soc. KOH 2014, 136, 11452 MnO_(x) film 1.0M 56349 J. Am. Chem. Soc. KOH 2012, 134, 17253. NiFeO_(x) film 1.0M >350 N/AJ. Am. Chem. Soc. NaOH 2013, 135, 16977. Fe—Ni oxides 1.0M >375 51 ACSCatal. KOH 2012, 2, 1793. Zn_(x)Co_(3-x)O₄ 1.0M 330 51 Chem. Mater.nanowire KOH 2014, 26, 1889. Ni_(x)Co_(3-x)O₄ 1.0M ~370 59-64 Adv.Mater. nanowire KOH 2010, 22, 1926.

The quick surpass of Ni—P over IrO₂ in OER activity was wellrationalized by their different Tafel slopes. As shown in FIG. 18(b),the linear fitting of their corresponding Tafel plots resulted in Tafelslopes of 49 mV/dec for Ni—P and 55 mV/dec for IrO₂, indicating a morefavorable OER kinetic rate of the former. Although NiO_(x) was reportedas a decent OER catalyst, its catalytic current did not take off until1.6 V vs RHE and its Tafel slope was 65 mV/dec, inferior to those ofNi—P and IrO₂. Besides great OER activity, our Ni—P film also featuredexcellent stability, as revealed by the overlap of its polarizationcurves before and after 1000 continuous cyclic voltammetric sweepswithin the potential ranges of 1.50-1.65 V (FIG. 18(c)) and 1.0-1.7 V(FIG. 19) vs RHE. A redox feature of Ni^(III/II) was observed around1.3-1.4 V vs RHE in the latter, consistent with those reportednickel-based OER electrocatalysts. (See, L. D. Burke, T. A. M. Twomey,J. Electroanal. Chem. 1984, 162, 101-119; and M. Wehrens-Dijksma, P. H.L. Notten, Electrochim. Acta 2006, 51, 3609-3621.)

The SEM image of a Ni—P film after a 2-h OER electrolysis at η=350 mV in1.0 M KOH is shown in FIG. 18(d). No apparent aggregates or particleswere observed; instead it still maintained an overall morphologyanalogous to those of the parent and post-HER Ni—P films. Despite nosignificant morphology change during OER, elemental analysis of thepost-OER film demonstrates that a large amount of oxygen species wereinvolved at least on the surface of the film (data not shown); whilenickel and phosphorous atoms were still evenly distributed over theentire film. It is well anticipated that nickel and phosphorous would beoxidized under anodic conditions of OER. Indeed, the high-resolution Ni2p XPS spectrum of the post-OER Ni—P film clearly presents the rise of ashoulder peak at ˜856.5 eV (FIG. 13(c)), which can be attributed tonickel oxides/hydroxides. However, the dominant peaks are still locatedat 852.9 and 870.1 eV, close to those of the as-prepared and post-HERfilms. Similarly, a peak at 133.8 eV ascribed to oxidized phosphorousspecies (e.g., phosphate) is observed in the high-resolution P 2p XPSspectrum (FIG. 13(d)). Nevertheless, another P 2p peak at a lowerbinding energy region of 129-131 eV is still present. In addition, Ramanspectra of the three Ni—P samples, as-prepared, post-HER, and post-OER,were collected and compared in FIG. 20. The Raman spectra of theas-prepared and post-HER samples are quite similar to each other,implying no substantial change in composition of the Ni—P film prior toand post HER. However, a prominent absorption peak was observed at500-600 cm⁻¹ for the post-OER Ni—P film, indicating the formation ofoxidized nickel species (e.g., nickel oxides and/or oxyhydroxides) onthe catalyst surface. Fourier transform infrared spectroscopy (FTIR) ofthe post-OER electrolyte solution did not show apparent oxidizedphosphorous species (e.g., phosphate), implying a very small amount ofoxidized phosphorous would be dissolved in the electrolyte solution(FIG. 21).

Given the aforementioned results, we concluded that the Ni—P film waspartially oxidized to nickel oxides/hydroxides during OER (most likelyon the film surface), while the bulk composition of the post-OER filmstill retained as the original Ni—P. It should be noted that core-shellstructure has been reported for Ni₂P nanowires as OER electrocatalysts,wherein the shell was mainly composed of nickel oxides/hydroxides whilethe core remained as Ni₂P. Elemental analysis of the post-OER filmresulted in the remaining amounts of nickel and phosphorous as 1.24 and0.29 mg/cm² with a Ni/P atomic ratio of 2.25. It is interesting to notethat the control sample NiO_(x) which might also possess a metallicnickel core and a nickel oxide shell was unable to compete with our Ni—Pin terms of both HER and OER activities, which undoubtedly prove thebeneficial role that phosphorous plays in water splittingelectrocatalysis.

Since the polarization-derived Tafel slopes aforementioned mightoverlook the impact of electron transport in the catalyst material onHER and OER performance, we further carried out detailed electrochemicalimpedance spectroscopy (EIS) studies to probe the intrinsic kinetics ofour Ni—P films. The EIS data for both HER and OER electrocatalysis canbe simulated by a semi-empirical electrical equivalent circuit modelshown in FIG. 22, (See J. Kibsgaard, T. F. Jaramillo, F. Besenbacher,Nat. Chem. 2014, 6, 248-253.) whereas R_(i) and R_(p) represent theuncompensated solution resistance and kinetics of interfacial chargetransfer, respectively. C_(dl) models the double layer capacitance.C_(s) and R_(s) in a parallel circuit simulate the relaxation of chargesassociated with adsorbed intermediates on catalyst surface. The Nyquistplots of Ni—P for HER at overpotentials of −40 to −130 mV are displayedin FIG. 23(a), together with solid fitting curves. The correspondingBode plots are shown in FIG. 24. The EIS-derived Tafel plot of log R_(p)vs overpotential (η) is included as an inset in FIG. 23(a). Analogous tothe pattern of polarization-derived Tafel plot (FIG. 15(b)), two kineticregions were observed, resulting in Tafel slopes of 33 and 98 mV/dec atlow and high overpotentials, respectively. This is again consistent withinsufficient proton supply when HER rate is very fast at highoverpotentials. For OER electrocatalysis, the EIS data were collected atoverpotentials of 270 to 360 mV and the Nyquist and Bode plots aredisplayed in FIGS. 23(b) and 25, respectively. The OER EIS results werealso simulated by the same equivalent electrical circuit successfullyand the solid fitting curves are included in FIG. 23(b) as well. FIG.23(b) inset shows the EIS-derived Tafel plot of OER catalyzed by ourNi—P film, rendering a slope of 52 mV/dec, which is in a good agreementwith the value resulting from the polarization-derived Tafel slope, 49mV/dec (FIG. 18(b)).

The OER mechanism of the transformed Ni—P film was furthered studied byan investigation conducted in KOH of various concentrations. Thepolarization curves were collected in 1.0-5.0 M KOH (FIG. 26). Thederived Tafel plots in FIG. 27(a) are nearly parallel to each other,indicating no kinetic change as the KOH concentration increased from 1.0to 5.0 M. When plotting the potential requirements at current densitiesof 5 and 10 mA/cm² versus the logarithm of the hydroxide activity([OH]_(a)), two linear plots were obtained in FIG. 27(b). Fitting ofthese two plots led to slopes of 51.92 (for 5 mA/cm²) and 51.88 (for 10mA/cm²) mV/dec, respectively. These slopes match thepolarization-derived (49 mV/dec, FIG. 18(b)) and resistance-derived (52mV/dec, FIG. 23(b)) Tafel slopes very well. According to Equation 1:

$\begin{matrix}{\left( \frac{\partial E}{\partial{\log \lbrack{OH}\rbrack}_{a}} \right) = {\left( \frac{\partial\log}{\partial{\log \lbrack{OH}\rbrack}_{a}} \right)\left( \frac{\partial E}{{\partial\log}\; i} \right)_{{\log {\lbrack{OH}\rbrack}}_{a}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

the close match of

$\frac{\partial E}{\partial{\log \lbrack{OH}\rbrack}_{a}}\mspace{14mu} {and}\mspace{14mu} \frac{\partial E}{{\partial\log}\; i}$

results in the unity of

$\frac{{\partial\log}\; i}{\partial{\log \lbrack{OH}\rbrack}_{a}}.$

(See Y. Surendranath, M. W. Kanan, D. G. Nocera, J. Am. Chem. Soc. 2010,132, 16501-16509.) In other words, the OER reaction rate catalyzed bythe Ni—P film is first order in the activity of hydroxide anion;therefore the limiting step is likely one hydroxide transfer, similar tothe reported mechanism of “CoPi”. Id.

From the above results, it is natural to anticipate that the Ni—P filmcan be employed as a bifunctional electrocatalyst for overall watersplitting. Indeed, when the as-prepared Ni—P films were used aselectrocatalysts for both anode and cathode (a Ni—P/Ni—P catalystcouple), a catalytic current was observed when the applied potential waslarger than 1.55V (FIG. 28). The rapid catalytic current density rapidlyexceeded 10 mA/cm² at 1.67 V. In addition, the Ni—P/Ni—P catalyst couplealso maintained excellent stability, as revealed by the nearly overlapof the polarization curves before and after 1000 continuous potentialcycles between 1.5 and 1.65 V (FIG. 28). Chronopotentiometry with acatalytic current of 10 mA/cm² was conducted for 24 h (FIGS. 29(a) and29(b)), showing fairly stable performance of the Ni—P/Ni—P catalystcouple.

In summary, potentiodynamically deposited Ni—P films have beendemonstrated to act as competent and bifunctional electrocatalysts foroverall water splitting. The Ni—P film is unique because of thefollowing reasons: (i) it is prepared via facile electrodeposition withlow-cost regents under an ambient condition and can be directly employedas an electrocatalyst for both HER and OER without any post treatment;(ii) the catalytic activity of the Ni—P film can rival thestate-of-the-art catalysts (i.e., Pt and IrO₂), requiring an η=−93 mVfor HER and η=344 mV for OER to reach a current density of 10 mA/cm²with corresponding small Tafel slopes of 43 and 49 mV/dec, respectively;(iii) it can be utilized as catalysts for both anode and cathode ofoverall water splitting catalysis under strongly alkaline condition withsuperior efficiency and strong robustness. Various characterization andanalytical techniques were applied to study the morphology andcomposition of the Ni—P film prior to and post electrocatalysis. It wasconcluded that the major component of the film is metallic nickel andnickel phosphide for the as-prepared and post-HER samples, whereas itwas partially oxidized to nickel oxides/hydroxides/phosphates on thesurface during OER. Kinetic analysis of its OER catalysis implied thelimiting step is the transfer of one hydroxide group. Different frommany reported hybrid systems, no conductive supports of high surfacearea, such as graphenes, carbon nanotubes, and nickel foams, wereinvolved in the current system. The introduction of catalyst supports ofhigh conductivity and large surface area will undoutedly further boostthe catalytic performance of the Ni—P film, which is under our currentpursuit.

In the following part of the present specification, numbered examplesare listed which are directed to and which define advantageousembodiments. Said examples and embodiments belong to the presentdisclosure and description. The embodiments, examples, and features aslisted, can separately or in groups, be combined in any manner to formembodiments belonging to the present disclosure.

Numbered Examples

1. A catalyst, comprising a conductive substrate coated with ametal-phosphorus-derived film, wherein the metal is selected from thegroup consisting of Manganese, Iron, Cobalt, Nickel, and Copper.

2. The catalyst of example 1, wherein the conductive substrate comprisesa material selected from the group consisting of copper, titanium,glassy carbon, fluorine-doped tin oxide, indium-doped tin oxide, tin,nickel, and stainless steel.

3. The catalyst of examples 1-2, wherein the metal-phosphorus-derivedfilm is electrodeposited.

4. The catalyst of examples 1-3, wherein the metal-phosphorus-derivedfilm comprises a cobalt-phosphorous-derived film.

5. The catalyst of examples 1-4, wherein the conductive substratecomprises a material selected from the group consisting of copper foil,nickel, and stainless steel.

6. The catalyst of examples 1-5, wherein the metal-phosphorus-derivedfilm has a phosphorus concentration of greater than 0 to about 50%.

7. The catalyst of examples 1-5, wherein the metal-phosphorus-derivedfilm has a phosphorus concentration of from about 5% to about 50%.

8. The catalyst of examples 1-5, wherein the metal-phosphorus-derivedfilm has a phosphorus concentration of from about 5% to about 20%.

9. The catalyst of examples 1-5, wherein the metal-phosphorus-derivedfilm has a phosphorus concentration of about 10%.

10. A method of producing hydrogen gas or oxygen gas, the methodcomprising:

providing a electrolysis solution, the electrolysis solution comprisingwater and an electrolyte;

inserting the catalyst of claim 1 into the electrolysis solution;

running an electric current through the catalyst.

11. The method of example 10, further comprising identifying andquantifying the hydrogen gas or oxygen gas.

12. The method of examples 10-11, further comprising collecting thehydrogen gas.

13. The method of examples 10-12, further comprising collecting theoxygen gas.

14. The method of examples 10-13, wherein the catalyst comprises acobalt-phosphorous-derived film on a conductive substrate.

15. The method of examples 10-14, wherein the conductive substratecomprises copper foil.

16. The method of examples 10-15, wherein the electrolysis solutioncomprises an alkaline electrolyte.

17. The method of examples 10-16, wherein the electrolysis solutions isselected from the group consisting of KOH and NaOH.

18. A method of producing a catalyst, the method comprising:

providing a working electrode and a counter electrode, the workingelectrode and counter electrode each comprising a conductive substrate;

electrodepositing on the working electrode a metal-phosphorus-derivedfilm, wherein the metal is selected from the group consisting ofManganese, Iron, Cobalt, Nickel, and Copper.

19. The method of example 18, wherein electrodepositing comprises:

-   -   submersing the working electrode and the counter electrode in a        deposition solution comprising a metal source that comprises        Manganese, Iron, Cobalt, Nickel, or Copper and a phosphorus        source, and    -   cycling a current through the working electrode and counter        electrode.

20. The method of examples 18-19, wherein:

the metal source comprises CoSO₄ and

the phosphorus source comprises NaH₂PO₂.

21. The method of examples 18-20, wherein the current is cycled betweena potential of −0.3 and −1.0 V vs Ag/AgCl.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also,various presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art, and are also intended to beencompassed by the following claims.

REFERENCES

The following references are hereby incorporated by reference in theirentireties:

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What is claimed is:
 1. A catalyst, comprising a conductive substratecoated with a metal-phosphorus-derived film, wherein the metal isselected from the group consisting of Manganese, Iron, Cobalt, Nickel,and Copper.
 2. The catalyst of claim 1, wherein the conductive substratecomprises a material selected from the group consisting of copper,titanium, glassy carbon, fluorine-doped tin oxide, indium-doped tinoxide, tin, nickel, and stainless steel.
 3. The catalyst of claim 1,wherein the metal-phosphorus-derived film is electrodeposited.
 4. Thecatalyst of claim 1, wherein the metal-phosphorus-derived film comprisesa cobalt-phosphorous-derived film.
 5. The catalyst of claim 1, whereinthe conductive substrate comprises a material selected from the groupconsisting of copper foil, nickel, and stainless steel.
 6. The catalystof claim 1, wherein the metal-phosphorus-derived film has a phosphorusconcentration of greater than 0 to about 50%.
 7. The catalyst of claim1, wherein the metal-phosphorus-derived film has a phosphorusconcentration of from about 5% to about 50%.
 8. The catalyst of claim 1,wherein the metal-phosphorus-derived film has a phosphorus concentrationof from about 5% to about 20%.
 9. The catalyst of claim 1, wherein themetal-phosphorus-derived film has a phosphorus concentration of about10%.
 10. A method of producing hydrogen gas or oxygen gas, the methodcomprising: providing a electrolysis solution, the electrolysis solutioncomprising water and an electrolyte; inserting the catalyst of claim 1into the electrolysis solution; running an electric current through thecatalyst.
 11. The method of claim 10, further comprising identifying andquantifying the hydrogen gas or oxygen gas.
 12. The method of claim 10,further comprising collecting the hydrogen gas.
 13. The method of claim10, further comprising collecting the oxygen gas.
 14. The method ofclaim 10, wherein the catalyst comprises a cobalt-phosphorous-derivedfilm on a conductive substrate.
 15. The method of claim 10, wherein theconductive substrate comprises copper foil.
 16. The method of claim 10,wherein the electrolysis solution comprises an alkaline electrolyte. 17.The method of claim 10, wherein the electrolysis solutions is selectedfrom the group consisting of KOH and NaOH.
 18. A method of producing acatalyst, the method comprising: providing a working electrode and acounter electrode, the working electrode and counter electrode eachcomprising a conductive substrate; electrodepositing on the workingelectrode a metal-phosphorus-derived film, wherein the metal is selectedfrom the group consisting of Manganese, Iron, Cobalt, Nickel, andCopper.
 19. The method of claim 18, wherein electrodepositing comprises:submersing the working electrode and the counter electrode in adeposition solution comprising a metal source that comprises Manganese,Iron, Cobalt, Nickel, or Copper and a phosphorus source, and cycling acurrent through the working electrode and counter electrode.
 20. Themethod of claim 19, wherein: the metal source comprises CoSO₄ and thephosphorus source comprises NaH₂PO₂.